Increasing mesh detail from existing model
Final result:



Overview:

Sadly you will need to retopologize your meshes. Happily it's not that hard and it's certainly good practice.

Main goal is to have quad only topology, which will allow us to use modifiers and have better control over the mesh itself.

There is no simpler method that I'm aware of to get high poly from low poly in order to prepare mesh for 3D printing.

Steps:




Remove vertices from the back of the mesh.
Merge two bottom vertices (I think only this particular mesh has this issue).
Delete one side of the mesh.
Add Mirror Modifier and select Axis accordingly to your mesh and check Clipping.
Delete inner edges.
Select two bottom vertices and hit F few times to fill in faces.
Make Loop Cut trough the mesh.
Place top vertices a little bit higher (use Edge Slide - G>G) to make nice Edge Loop and add another Loop Cut for sharp center (press E when sliding edge to make it even with left/straight edge).





Add Solidify and Subsurface Modifiers.


Final thoughts:

Now you can add as many Loop Cuts as you want to have sharper/smoother edges. You can use Mean Crease to sharpen them even more. Meshes from your file may produce some issue while printing. It's better to make an effort and prepare good mesh then loose time and money while printing.

Blend file: